```
public class Solution {
public boolean isPowerOfFour(int num) {
if(num<0 || num==0) return false;
if(num==1 || num==4) return true;
String strNum=Integer.toBinaryString(num);
if(strNum.length()<3 || (strNum.length()-1)%2!=0) return false;
strNum=strNum.replace("00","");
return strNum.equals("1");
}
}
```

if we take a close look at all the numbers that are power of four, it is obivious that those numbers are in 100^n (n is a non-negative integer) pattern.

So, after converting *num* to binary-string and replacing all the *"00"* with *""*, we can easily find out whether num is a power of four by determine if the left string equals *"1"*.